A battery pack is used as a power source for a portable personal computer called a laptop type or notebook type personal computer, a mobile information terminal, a mobile phone and a mobile-oriented electronic device such as a mobile-oriented music player. Many battery packs used for these electronic devices may employ a combined-cell battery, and the electronic device is mounted with a combined-cell battery pack including a combination of the single-cells. A reason why so lies in satisfying voltage or current conditions requested of these electronic devices. In this type of battery pack, it is possible to provide a battery pack having a variety of power source voltages and current capacities by arranging the cells in series or in parallel and further combining these cells.
By the way, the notebook type personal computer involves using a lithium (Li) ion secondary battery in many cases. In the case of using the lithium (Li) ion secondary battery, however, it is required that safety of the battery be sufficiently taken into consideration. Therefore, a protection circuit is mounted in the battery pack, monitors each of cell blocks and performs, when abnormality occurs in voltage, its protective function matching with this situation (refer to, e.g., Patent document 1).
This type of prior art discloses a configuration that the battery incorporates a plurality of cell units in parallel, in which a plurality of battery cells is connected in series. Then, when a voltage of any one of the cell units deviates from a predetermined range (falling into an abnormal status), this abnormal cell unit can be separated. In this case, the cell units other than the separated cell unit are possible of charging/discharging operations and can be also used continuously.    [Patent document 1] Japanese Patent Publication No. 3330517    [Patent document 2] Japanese Patent Laid-Open Publication No. H07-22009    [Patent document 3] Japanese Patent Laid-Open Publication No. 2000-102185    [Patent document 4] Japanese Unexamined Patent Publication No. 2005-528070    [Patent document 5] Japanese Patent Laid-Open Publication No. 2004-31273